Lucas mostly just wanted to get across the idea that plucky underdogs can overcome a technologically superior enemy if they truly believe in what they’re fighting for rather than intangible and abstract goals that they’re apathetic about like “stop whatever this thing called communism is in some other country I’ve barely heard of halfway across the world”, which is what his main takeaway from Vietnam was.
George certainly didn’t object to Kirshner making the Empire all Brits and the rebels all Americans (and having that stick!) as the same basic idea got across.